An optical modulator, such as an electro-optical modulator, may modulate a beam. The optical modulator may include a z-cut lithium niobate (LN) substrate, a set of radio frequency (RF) signal electrodes, a set of ground electrodes, and a set of coplanar waveguides to modulate the beam. The set of coplanar waveguides can be part of a Mach-Zehnder (MZ) interferometer. For example, the optical modulator may be configured to use a nested quad-parallel MZ interferometer based modulator (QPMZM) waveguide topology. An optical modulator with coplanar waveguides may cause RF dielectric modes and parallel-plate modes to be excited within the substrate. RF power in the fundamental electromagnetic mode of the set of coplanar waveguides can couple to the substrate modes, which may cause RF loss and/or resonant dips in an optical modulator frequency response. The resonance frequencies of the substrate modes are based on the dimensions of the substrate. For example, the resonance frequencies shift toward higher frequencies as the dimensions of the substrate are reduced toward smaller dimensions.
Thus, in some cases, reducing the substrate thickness causes the RF loss and the resonances to shift to a higher frequency, which may be outside of an operating frequency range of the optical modulator, thereby obviating a negative impact to performance of the optical modulator. However, reducing a substrate thickness may reduce mechanical durability of the optical modulator and may increase a difficulty of manufacture by reducing mechanical rigidity of the optical modulator. Accordingly, it would be advantageous if an optical modulator could be configured to suppress parasitic RF modes (e.g., which cause the RF loss) with a substrate greater than a threshold thickness to ensure mechanical rigidity and mechanical durability.